1. Field of the Invention
The present invention relates to processing substrates in semiconductor fabrication, and more particularly to a method of adjusting semiconductor processing equipment to optimize uniformities within a wafer.
2. Description of the Related Art
Rapid thermal processing (RTP), including annealing, oxidation, and chemical vapor deposition (CVD), is a powerful technique for fabrication of integrated circuit devices. CVD is useful in the formation of semiconductor materials (e.g., single-crystalline, poly-crystalline and amorphous Si, SiGe, SiGe:C and Si:C alloys) in addition to dielectric oxides and nitrides as well as conductors, such as aluminum, copper, tungsten, titanium nitride, and numerous other metals and metal compounds. The move toward smaller devices and larger wafers has put more emphasis on the need for wafer uniformity during RTP. In processing wafers in a thermal reactor, it is therefore important to optimize wafer heating and other settings to maximize uniformity of the wafer in order to minimize variations in process results (e.g., thickness, resistivity, germanium content, carbon content).
A thermal reactor common in the art includes two banks of heating lamps arranged in orthogonal or crossed directions above and below a susceptor holding a semiconductor substrate. Each of the upper and lower surfaces of the substrate faces one of the two banks of heating lamps. A controller within the thermal reactor adjusts the relative power to each lamp to maintain a desired temperature during wafer processing. The controller receives signals indicative of wafer temperature from temperature sensors, such as pyrometers or thermocouples. The controller uses the temperature signals to make adjustments to the heating lamps to ensure uniformity in temperature across the wafer surface.
To place the reactor in an operating condition, the heating lamps require various adjustments prior to processing of the wafer. For example, one may need to adjust the position and the orientation of the heating lamps relative to a wafer in the semiconductor processing chamber. Similarly, each temperature sensor generally requires a controller offset adjustment (for a conventional proportional-integral-derivative or PID controller) to account for the variances in the sensitivity and accuracy of each sensor, in addition to its position relative to the wafer.
While efforts have been made to measure thickness to make these adjustments as described by Shaper et al. in J. Electrochem Soc., 143, 374 (1996), such efforts are cumbersome, time-consuming, and complicated, requiring numerous expensive depositions, thickness measurements, optimization studies, and curve fitting. Currently, these adjustments to place the thermal reactor in operating condition are tedious and costly. For example, in a thermal reactor used for silicon epitaxy, crystal structure defects, such as slip planes, dislocations, and stacking faults, may result from temperature variations across the surface of the wafer. These defects and color variations (or other thickness measurements) are the only evidence available to a maintenance technician to identify and make wafer heating changes to eliminate non-uniformities. U.S. Pat. No. 6,126,744 describes a method of adjusting a reactor by forming a layer on a monitor wafer in the reactor and then observing the color variations from above the layer. The adjustments to the reactor are determined in accordance with the color variations, which correspond to thickness variations that result from surface temperature variations. However, such thickness or defect measurements provide only crude indications as to the location or the magnitude of adjustments to the temperature controllers that will address the non-uniformities.
Thus, equipment maintenance technicians employ a trial and error process to make the necessary adjustments to minimize wafer surface temperature differences. In another example, a slip plane defect in a silicon epitaxy layer appears as a line when viewed on an interference/contrast microscope using 50× magnification. The maintenance technician inspects the defective wafer to identify the slip plane defects and their locations to make adjustments. After completing the adjustments, the maintenance technician processes another production wafer to surmise the effectiveness of the adjustments. This second production wafer usually includes different slip plane defects as compared to those of the first production wafer, particularly where interplay among different temperature control zones make adjustments difficult to predict. The maintenance technician makes additional adjustments to compensate for the different slip plane defects and continues this iterative trial and error process until an acceptable density of defects results after processing a production wafer. The number of samples required often depends upon the experience and choices of the technicians. The inefficiency of this trial and error process results in loss of valuable production time as well as production wafers.
Maintenance technicians may also use an instrumented wafer to minimize temperature variation. An instrumented wafer is a wafer having a plurality of thermocouples mounted through its top surface. Each thermocouple provides to a data acquisition device signals indicative of the wafer temperature at the mounting location. Wafer processing conditions are simulated in the thermal reactor while the thermocouples provide temperature signals to the data acquisition device for analysis. Due to the connected cables/wires, this is only possible with a non-rotating wafer. A maintenance technician then analyzes the temperature profile data to determine adjustments needed to minimize surface temperature variations.
Unfortunately, the instrumented wafer thermocouples often fail after one or two hours of use under standard wafer processing conditions. Additionally, the thermocouples often provide false readings under the hydrogen flows of standard wafer processing conditions. The presence of metal wiring associated with the thermocouples creates an unacceptable risk of contamination to many semiconductor manufacturers. Some semiconductor manufacturers require a thermal reactor reconstruction or chamber clean after any use of an instrumented wafer. Moreover, the frequent failure of thermocouples and false readings decrease the reliability of the temperature data acquired through the thermocouples and consequently the effectiveness of any adjustments made in response to such data. It can be very costly for a semiconductor manufacturer to use multiple instrumented wafers and to reconfigure the thermal reactor. Efforts have been made to minimize temperature non-uniformity in non-rotating wafers, as described in Acharya et al., IEEE Transactions on Semiconductor Manufacturing, 14, 218 (2001).
Similar difficulties exist for optimizing numerous other parameters for semiconductor processing, such as gas flows and distribution, pressure, etc. It is therefore highly desirable to use a simple and robust uniformity optimization technique to attain the desired process uniformity without losing valuable production time and to minimize the number of monitor wafers used during system adjustment.